The present invention generally relates to micro electromechanical systems or so-called MEMS. More particularly, the present invention relates to a switching device comprising a mobile element that is able to move back and forth along a defined trajectory between a zero position and at least one predetermined switching position, an elastic member connecting the mobile element to a base, and at least one stationary actuating electrode located in the vicinity of the said at least one predetermined switching position. The present invention also relates to a method for operating such a switching device and a switching unit that comprises at least one such switching device.
A very wide variety of micromechanical structures that influence or switch, respectively, the paths of optical beams are known. The purpose of such structures is the control, processing, and/or storage of data from pixel-based images.
European Patent Application No. 0 510 629 discloses a deflectable device based on so-called deformable mirror devices, or DMDs, which is also used as a shutter device for selectively interrupting or altering the passage of a light beam. As for other types of DMDs, the device is provided with a controllable screen plate which can either rotate around an axis or move towards the substrate""s plane in a piston-like fashion by means of adequate torsion beams, cantilever beams or hinges. Other examples of DMDs may for instance be found in U.S. Pat. No. 4,229,732 and U.S. Pat. No. 5,142,405.
U.S. Pat. No. 5,794,761 discloses a switching device that may be used as a micro-shutter for optical applications. FIG. 1 is a schematic view of this switching device. According to this document, mobile element 2, or swinging element, is able to move between two end positions A and B to selectively interrupt the passage of a light beam through an opening 7 (located in end position B) provided in the substrate of the switching device. Electrodes 5, 6 are placed in the vicinity of end positions A and B. A control circuit 8 is provided to generate electrostatic forces of attraction and/or repulsion for switching the swinging element 2 between the two end positions. The swinging element 2 is attached to the substrate in an elastic manner (for example by means of a flexible beam 3 which is clamped at one of its end to an anchoring point or base 4). More particularly, according to this document, beam 3 is designed in such a way that an elastic return force which tends towards a zero position O has, in an essential amplitude range, a higher value than the electrostatic force of attraction generated by the control circuit and the electrodes. Accordingly, the characteristics of the switching devices, such as its response time, are mainly determined by the mechanical properties of the structure.
The article xe2x80x9cElectrostatic Microshutter Arrays in Polysiliconxe2x80x9d, G. Perregaux et al., published in xe2x80x9cCSEM Scientific and Technical Report 1999xe2x80x9d, page 99, discloses an improvement of the above-mentioned switching device. FIG. 2 is a schematic view of this other switching device. Electrodes 5, 6 are provided along the whole length of the flexible beam 3 which connects the swinging element 2 to the substrate. Stoppers 9 are further provided on both sides of flexible beam 3 along its length in order to avoid short-circuits between the beam and the electrodes.
Other types of optical micro-shutter devices may be found in U.S. Pat. No. 4,383,255 or U.S. Pat. No. 4,564,836.
According to all of the above prior art solutions, the switching device is actuated by means of attractive forces, i.e. the electrodes are disposed laterally with respect to the mobile element""s trajectory so as to produce electrostatic forces which are substantially normal to the cooperating edges of the electrodes and of the swinging element. Stoppers are therefore typically required to limit the displacement of the mobile element as well as prevent any short-circuit between the electrodes and the mobile element.
The contact between the polysilicon surface (or other material) and the stoppers may lead to extensive wear and debris production and can eventually lead to sticking of the mobile element. The so-called xe2x80x9cstiction problemxe2x80x9d (which also encompasses problems due to capillarity forces or organic and non-organic contamination) Is a well-known and very actual and critical problem with micromechanical switching devices. In the small dimensions, the sticking effect increases with the inverse of the scale down factor, with a preponderance effect due to humidity, surface trapped charges or other proximity forces (such as quantum Van de Walls forces). These sticking forces are in the same order of magnitude than the activation forces (around 0.2 xcexcN/xcexcm2). They therefore have significant influence on the reliability of such systems.
In order to overcome this problem, a solution is to conceive structures without stoppers. Without stoppers the control of the displacements of the switching device""s mobile element highly depend on the drive level or on an auto-blocking elastic effect such as buckling. Structures using the buckling effect are however very sensitive to technology tolerances and are therefore expensive and complicated to manufacture.
Another solution is to chemically treat the device surfaces and for instance coat the device with an additional layer such as a polymer. This solution however increases the fabrication complexity of the device as well as its costs. In addition, excessive wear of this coating layer can nevertheless give rise to the above stiction problem.
There exists therefore a need for simpler and more reliable solutions to overcome the above-mentioned stiction and short-circuit problems. A principal object of the present invention is thus to provide a solution that is not prone to stiction problems and that is as much as possible independent of the drive and technological tolerances to control the displacements of the mobile element of the switching device.
Another object of the present invention is to provide a solution that is not unnecessarily complicated to manufacture.
A secondary object of the present invention is to provide a switching device that can be arranged in a matrix configuration.
Accordingly, there is provided a switching device the features of which are listed in claim 1.
There is also provided a switching unit comprising a number of such switching device, as well as a method for actuating a switching device the features of which are listed in claim 21.
Other advantageous embodiments of the invention are the object of the dependent claims.
According to the present invention, and in contrast to prior art solutions, the actuating electrodes are not disposed to act laterally on the mobile element so as to produce attractive electrostatic forces but act on a frontal edge of the mobile element (that is an edge which is substantially parallel to the mobile element""s trajectory). This driving principle may be called xe2x80x9cEdge Electrostatic Drivexe2x80x9d, that is the movement of the mobile element is not caused by attractive forces generated by the actuating electrodes (as in the prior art solutions), but is rather caused by lateral forces which are substantially parallel to the cooperating frontal edges of the stationary electrodes and of the electrode of the mobile element.
In the scope of the present invention, an attractive electrostatic force shall mean an electrostatic force which tends to bring the cooperating edges of the electrodes nearer (or, conversely, which tends to separate these cooperating edges), whereas a lateral electrostatic force shall mean an electrostatic force which tends to align the cooperating edges of the electrodes one with respect to the other.
According to the present invention, the switching device can truly be designed as a contact-less structure which is not affected by the above-mentioned stiction problems. Indeed, stoppers are not anymore required to define the end positions of the mobile element. As a matter of fact, the mobile element is auto-stabilized in its selected position due to the inversion of the forces resulting from the sum of the lateral electrostatic forces and mechanical spring forces exerted on the device""s mobile element.
According to the present invention, the geometry of the device is selected to enhance the contributions of lateral electrostatic forces. Furthermore, In contrast to prior art solutions, the device""s switching positions are determined by an equilibrium point between the lateral electrostatic forces produced by the electrodes on the device""s mobile element and mechanical spring forces produced by the elastic member that connects the mobile element to the device""s base member. The force resulting from the sum of these lateral electrostatic force and mechanical spring force presents an inversion point around this equilibrium point
More particularly, according to one embodiment of the invention, the switching device is used as a shutter device for selectively interrupting the passage of a light beam, the switching device being provided with an opening, which is either open or dosed by a screen plate.
According to another embodiment of the invention, the switching device comprises an optical deflecting member disposed perpendicularly to the plane of movement of the switching device. Such device can be used as a shutter or mirror for light beams produced parallel to the switching device""s plane.
According to another embodiment of the invention, the switching device is used as a deflectable device and performs the operation of so-called xe2x80x9cdeformable mirror devicesxe2x80x9d or DMDs, the switching device being provided with a controllable screen plate comprising a reflective surface, this controllable screen plate being rotated around at least one axis of rotation.
It should be pointed out that the present invention can advantageously be applied in optical application to interrupt, modulate or control the passage of light beams but is however not limited to that particular type of applications.